Alkyne alkylidyne metal complexes

The chemistry of CR fragments ligating metal centers has been a topic of considerable interest since the discovery of the first alkylidyne-metal complexes in E.O. Fischer s Laboratory in 1973 (1). These ligands have been implicated in Fischer-Tropsch reactions (2) and in alkyne metathesis (5). Moreover, the isolation of stable compounds containing carbon-metal triple bonds completed the matrix of bond types represented here ... 

Two commonly used synthetic methodologies for the synthesis of transition metal complexes with substituted cyclopentadienyl ligands are important. One is based on the functionalization at the ring periphery of Cp or Cp metal complexes and the other consists of the classical reaction of a suitable substituted cyclopentadienyl anion equivalent and a transition metal halide or carbonyl complex. However, a third strategy of creating a specifically substituted cyclopentadienyl ligand from smaller carbon units such as alkylidynes and alkynes within the coordination sphere is emerging and will probably find wider application [22]. 

Scheme 1. Alkene-and alkyne-metal complexes may be formulated with metallacyclo-propane (A) or -propene (B) structures, respectively. Similarly, complexation of a metal atom with an alkylidene or alkylidyne group gives rise to a dimetallacyclopropane (C) or -propene (D) ring system.

Section III of the review is concerned with alkylidene and alkylidyne halocarbonyl complexes, and a review on recent advances in the chemistry of metal-carbon triple bonds by Mayr and Hoffmeister in 1991 contains material of interest.6 Section IV is concerned with halocarbonyl alkyne and alkene complexes described from 1987 to mid-1995, because an exten-... 

Just as alkylidenes and alkylidynes may bridge M-M bonds, vinyli-denes may also be employed to support metal-metal bonds. This approach has included the addition of preformed terminal vinylidene complexes to unsaturated metal complexes or, occasionally, the direct assembly from terminal alkynes with unsaturated metal complex precursors (Figure 5.50). 

The metathesis of alkynes by metal alkylidyne complexes is now a well-established process. The reaction is useful for the catalytic metathesis of alkynes as well as for the synthesis of new metal alkylidyne complexes. The general electronic and steric factors favoring the metathesis reaction were discussed in recent reviews by Schrock (6,7). 

The triscarboxylate alkylidyne complexes 280 form cyclopropenyl metal complexes with alkynes [Eq. (201)] (752). The complexes 281 do not revert... 

Despite their obvious similarity to alkenes and alkynes, transition-metal alkylidene and alkylidyne complexes have not been used as building blocks for the synthesis of lowdimensional materials analogous to polyenes and polyynes. We have begun to explore the syntheses, structures, and properties of conjugated complexes and polymers derived from metal-alkylidyne complexes as part of our effort to develop the chemistry of transition-metal analogues of conjugated organic compounds. 

The chemistry of alkylidene and alkylidyne complexes of early transition metals was developed by Schrock and co-workers and these complexes turned out to be of crucial importance to alkene and alkyne metathesis. Initially their research focused on tantalum complexes of the type CpTaCEIE, which after a-elimination (Figure 16.6) led to alkylidene complexes Cp(R)Cl2Ta=CHR [11]. 

The reversible [2+2] cycloaddition of metal alkylidyne or Fischer-type metal carbyne complexes remains the only general methodology for the synthesis of metallacyclobutadiene complexes. Recent literature revolves principally around the heavier group 6 metals and the investigation of intermediates in catalytic alkyne metathesis (Scheme 25 Equation 45) <1996CHEC-II(lb)887> (W <2005OM4684>, Mo <2003JOM56>). 

Several factors affect the nature of the products in a reaction between a transition metal cluster and an alkyne or alkene. In this section, the various synthetic routes to alkyne or alkene-substituted clusters will be presented, and these will be used to analyze the changes in reactivity of the cluster systems when one or more of the important reaction parameters is altered. In order to simplify the discussion, tri-, tetra-, and higher nuclearity clusters will be treated separately. Finally, in this section, there is a brief description of the chemistry of alkylidyne-substituted clusters since synthetic routes to alkyne-containing complexes may involve these species. 

Osmium carbyne (see Carbyne Complexes) or aUcylidyne complexes have a triple bond between the metal and the carbon atom of the ligand. Carbyne complexes are related to singlet carbenes. They are analogous to linear nitrosyl (see Nitrosyl Complexes) complexes and the osmium is usually in a lower oxidation state. Alkylidyne complexes are related to triplet carbenes and the bonding between the osmium and the carbon atom is similar to the C-C bond in an alkyne. 

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